Buy a roll of 1/8 sheet cork at an auto parts store. You must trim at the body a bit with a knife, to cut away a bit of the crimp they put in the edge of the hole that the plug of the valve fits into.
Then put the handle in a vise, and pull and twist the body, CAREFULLY, so you don't break the handle off, and you will pull the plug of the valve, and the retaining washer ( which the crimp in the body holds) out.
Clean out the body with a round plumbers wire brush. Sand the plug LIGHTLY with 600 sandpaper. Then you cut out a piece from the cork sheet that measures 1 29/32" x 1 1/6". Make sure that your cut piece is square and accurate.
Grease up one side with wheel bearing grease, and roll it into a more or less cylinder, an shove it, (with the open seam to the side of the body) into the body. Be sure to get it all the way in, and square. Then grease up the plug and press it in. It will take a good bit of force, and I do it with the aid of a vise.
When you get it all the way in, take a small screwdriver, and a light hammer, and crimp the body just a bit, to hold the retaining washer in. Then turn the petcock to the on position. (it will take a good deal of force at first, 'cause it WILL be tight.) and find a drill bit that will fit down inside the brass supply tubes, and CAREFULLY drill the cork out of the OPEN hole and the hole on the output side too. Blow out the crumbs of cork with compressed air, and turn the petcock to the reserve position, and do it again.
After you put it back in use, if you ever take it off, do not allow it to ever dry out. Put the petcock in a jar of gasoline, with a tight lid.